Susan L. Beck

Subduction and collision processes in the Central Andes constrained by converted seismic phases

The Central Andes are the Earths highest mountain belt formed by ocean–continent collision. Most of this uplift is thought to have occurred in the past 20 Myr, owing mainly to thickening of the continental crust, dominated by tectonic shortening. Here we use P-to-S (compressional-to-shear) converted teleseismic waves observed on several temporary networks in the Central Andes to image the deep structure associated with these tectonic processes. We find that the Moho (the Mohorovičić discontinuity—generally thought to separate crust from mantle) ranges from a depth of 75 km under the Altiplano plateau to 50 km beneath the 4-km-high Puna plateau. This relatively thin crust below such a high-elevation region indicates that thinning of the lithospheric mantle may have contributed to the uplift of the Puna plateau. We have also imaged the subducted crust of the Nazca oceanic plate down to 120 km depth, where it becomes invisible to converted teleseismic waves, probably owing to completion of the gabbro–eclogite transformation; this is direct evidence for the presence of kinetically delayed metamorphic reactions in subducting plates. Most of the intermediate-depth seismicity in the subducting plate stops at 120 km depth as well, suggesting a relation with this transformation. We see an intracrustal low-velocity zone, 10–20 km thick, below the entire Altiplano and Puna plateaux, which we interpret as a zone of continuing metamorphism and partial melting that decouples upper-crustal imbrication from lower-crustal thickening.

Crustal-thickness variations in the central Andes

We estimated the crustal thickness along an east-west transect across the Andes at lat 20°S and along a north-south transect along the eastern edge of the Altiplano from data recorded on two arrays of portable broadband seismic stations (BANJO and SEDA). Waveforms of deep regional events in the downgoing Nazca slab and teleseismic earthquakes were processed to isolate the P-to-S converted phases from the Moho in order to compute the crustal thickness. We found crustal-thickness variations of nearly 40 km across the Andes. Maximum crustal thicknesses of 70–74 km under the Western Cordillera and the Eastern Cordillera thin to 32–38 km 200 km east of the Andes in the Chaco Plain. The central Altiplano at 20°S has crustal thicknesses of 60 to 65 km. The crust also appears to thicken from north (16°S, 55–60 km) to south (20°S, 70–74 km) along the Eastern Cordillera. The Subandean zone crust has intermediate thicknesses of 43 to 47 km. Crustal-thickness predictions for the Andes based on Airy-type isostatic behavior show remarkable overall correlation with observed crustal thickness in the regions of high elevation. In contrast, at the boundary between the Eastern Cordillera and the Subandean zone and in the Chaco Plain, the crust is thinner than predicted, suggesting that the crust in these regions is supported in part by the flexural rigidity of a strong lithosphere. With additional constraints, we conclude that the observation of Airy-type isostasy is consistent with thickening associated with compressional shortening of a weak lithosphere squeezed between the stronger lithosphere of the subducting Nazca plate and the cratonic lithosphere of the Brazilian craton.

Rupture Characteristics of the Deep Bolivian Earthquake of 9 June 1994 and the Mechanism of Deep-Focus Earthquakes

The Mw = 8.3 deep (636 kilometers) Bolivian earthquake of 9 June 1994 was the largest deep-focus earthquake ever recorded. Seismic data from permanent stations plus portable instruments in South America show that rupture occurred on a horizontal plane and extended at least 30 by 50 kilometers. Rupture proceeded at 1 to 3 kilometers per second along the down-dip azimuth of the slab and penetrated through more than a third of the slab thickness. This extent is more than three times that expected for a metastable wedge of olivine at the core of the slab, and thus appears to be incompatible with an origin by transformational faulting. These large events may instead represent slip on preserved zones of weakness established in oceanic lithosphere at the Earths surface.

Lithospheric‐scale structure across the Bolivian Andes from tomographic images of velocity and attenuation for P and S waves

We have developed a three-dimensional, lithospheric-scale model across the Bolivian Andes at ∼20°S, based on tomographic images of velocity and attenuation for both P and S waves. Observations of travel time and attenuation for this study are from regional, mantle earthquakes in the subducted Nazca plate recorded on a portable, broadband seismic array (Broadband Andean Joint Experiment and Seismic Exploration of the Deep Andes) in Bolivia and Chile. The shallow mantle under the Altiplano from ∼18°S to ∼21°S is high-velocity and moderately high Q (Vp ≈ 8.3,Vs ≈ 4.7, Qp ≈ 500, and Qs ≈ 200), suggesting lithospheric mantle. High-velocity material in the Altiplano extends to a depth of ∼125–150 km. The shallow mantle of the Western Cordillera is characterized by high Vp/Vs (∼1.83), suggesting a correlation between Vp/Vs and arc volcanism. Seismic velocity in the Western Cordillera mantle is, on average, only slightly reduced from global averages; however, velocity and attenuation anomalies are locally strong ( Vp ≈ 7.8, Vs ≈ 4.3, Qp ≈ 200, and Qs ≈ 100), consistent with partial melt conditions. Under the Los Frailes volcanic field, in the Eastern Cordillera, shallow mantle velocity and Q decrease drastically from the neighboring Altiplano ( Vp ≈ 7.8, Vs ≈ 4.3, Qp ≈ 300, Qs ≈ 100); however, high Vp/Vs is not as pervasive as it is in the Western Cordillera. We believe that slab-derived water, and perhaps other volatiles, strongly influence the Western Cordillera, while the Eastern Cordillera low-velocity region is more affected by partial melt and/or compositional changes. Average velocity and Q in the shallow mantle across the Bolivian Andes, where the tomographic images are best resolved, are significantly higher than in most mantle wedge environments where corresponding images are available. This is likely the result of a compressional “back arc” setting in the Andes. This implies that lithospheric shortening and thickening associated with the formation of the Andes has profoundly influenced the shallow mantle structure across the range. Shallow mantle structure is locally influenced by the subduction processes, particularly under the Western Cordillera; however, the differing volcanism and seismic character under the two Cordilleras suggest that the volcanic process in the Eastern Cordillera may be distinct from arc volcanism. Tertiary volcanism in the Eastern Cordillera is located in the region where mantle shortening is suspected to be greatest. Both the timing and location of volcanism are consistent with upward migration of mantle wedge asthenosphere following the removal of over thickened lithosphere.

The Rupture Process and Tectonic Implications of the Great 1964 Prince William Sound Earthquake

We have determined the rupture history of the March 28, 1964, Prince Williams Sound earthquake (Mw=9.2) from long-period WWSSNP-wave seismograms. Source time functions determined from the long-periodP waves indicate two major pulses of moment release. The first and largest moment pulse has a duration of approximately 100 seconds with a relatively smooth onset which reaches a peak moment release rate at about 75 seconds into the rupture. The second smaller pulse of moment release starts at approximately 160 seconds after the origin time and has a duration of roughly 40 seconds. Because of the large size of this event and thus a deficiency of on-scale, digitizableP-wave seismograms, it is impossible to uniquely invert for the location of moment release. However, if we assume a rupture direction based on the aftershock distribution and the results of surface wave directivity studies we are able to locate the spatial distribution of moment along the length of the fault. The first moment pulse most likely initiated near the epicenter at the northeastern down-dip edge of the aftershock area and then spread over the fault surface in a semi-circular fashion until the full width of the fault was activated. The rupture then extended toward the southwest approximately 300 km (Ruff andKanamori, 1983). The second moment pulse was located in the vicinity of Kodiak Island, starting at ∼500 km southwest of the epicenter and extending to about 600 km. Although the aftershock area extends southwest past the second moment pulse by at least 100 km, the moment release remained low. We interpret the 1964 Prince William Sound earthquake as a multiple asperity rupture with a very large dominant asperity in the epicentral region and a second major, but smaller, asperity in the Kodiak Island region.The zone that ruptured in the 1964 earthquake is segmented into two regions corresponding to the two regions of concentrated moment release. Historical earthquake data suggest that these segments behaved independently during previous events. The Kodiak Island region appears to rupture more frequently with previous events occurring in 1900, 1854, 1844, and 1792. In contrast, the Prince William Sound region has much longer recurrence intervals on the order of 400–1000 years.

Anisotropy and mantle flow in the Chile-Argentina subduction zone from shear wave splitting analysis

[1] We examine shear wave splitting in teleseismic phases to observe seismic anisotropy in the South American subduction zone. Data is from the CHARGE network, which traversed Chile and western Argentina across two transects between 30� S and 36� S. Beneath the southern and northwestern parts of the network, fast polarization direction (j) is consistently trench-parallel, while in the northeast j is trench-normal; the transition between these two zones is gradual. We infer that anisotropy sampled by teleseismic phases is localized within or below the subducting slab. We explain our observations with a model in which eastward, Nazca-entrained asthenospheric flow is deflected by retrograde motion of the subducting Nazca plate. Resulting southward flow through this area produces N-S j observed in the south and northwest; E-W j result from interaction of this flow with the local slab geometry producing eastward mantle flow under the actively flattening part of the slab. INDEX TERMS: 7203 Seismology: Body wave propagation; 7218 Seismology: Lithosphere and upper mantle; 8123 Tectonophysics: Dynamics, seismotectonics; 8150 Tectonophysics: Plate boundary—general (3040); 9360 Information Related to Geographic Region: South America. Citation: Anderson, M. L., G. Zandt, E. Triep, M. Fouch, and S. Beck (2004), Anisotropy and mantle flow in the Chile-Argentina subduction zone from shear wave splitting analysis, Geophys. Res. Lett., 31, L23608, doi:10.1029/ 2004GL020906.

Shear wave anisotropy beneath the Andes from the BANJO, SEDA, and PISCO experiments

We present the results of a detailed shear wave splitting analysis of data collected by three temporary broadband deployments located in central western South America: the Broadband Andean Joint experiment (BANJO), a 1000-km-long east-west line at 20°S, and the Projecto de Investigacion Sismologica de la Cordillera Occidental (PISCO) and Seismic Exploration of the Deep Altiplano (SEDA), deployed several hunderd kilometers north and south of this line. We determined the splitting parameters Φ (fast polarization direction) and δt (splitting delay time) for waves that sample the above- and below-slab regions: teleseismic * KS and S, ScS waves from local deep-focus events, as well as S waves from intermediate-focus events that sample only the above-slab region. All but one of the * KS stacks for the BANJO stations show E-W fast directions with δt varying between 0.4 and 1.5 S. However, for * KS recorded at most of the SEDA and PISCO stations, and for local deep-focus S events north and south of BANJO, there is a rotation of Φ to a more nearly trench parallel direction. The splitting parameters for above-slab paths, determined from events around 200 km deep to western stations, yield small delay times (≤0.3 s) and N-S fast polarization directions. Assuming the anisotropy is limited to the top 400 km of the mantle (olivine stability field), these data suggest the following spatial distribution of anisotropy. For the above-slab component, as one goes from east (where * KS reflects the above-slab component) to west, Φ changes from E-W to N-S, and delay times are substantially reduced. This change may mark the transition from the Brazilian craton to actively deforming (E-W shortening) Andean mantle. We see no evidence for the strain field expected for either corner flow or shear in the mantle wedge associated with relative plate motion. The small delay times for above-slab paths in the west require the existence of significant, spatially varying below-slab anisotropy to explain the * KS results. The implied anisotropic pattern below the slab is not easily explained by a simple model of slab-entrained shear flow beneath the plate. Instead, flow induced by the retrograde motion of the slab, in combination with local structural variations, may provide a better explanation.

Historical 1942 Ecuador and 1942 Peru subduction earthquakes and earthquake cycles along Colombia-Ecuador and Peru subduction segments

Two large shallow earthquakes occurred in 1942 along the South American subduction zone inclose proximity to subducting oceanic ridges: The 14 May event occurred near the subducting Carnegie ridge off the coast of Ecuador, and the 24 August event occurred off the coast of southwestern Peru near the southern flank of the subducting Nazca ridge. Source parameters for these for these two historic events have been determined using long-periodP waveforms,P-wave first motions, intensities and local tsunami data.We have analyzed theP waves for these two earthquakes to constrain the focal mechanism, depth, source complexity and seismic moment. Modeling of theP waveform for both events yields a range of acceptable focal mechanisms and depths, all of which are consistent with underthrusting of the Nazca plate beneath the South American plate. The source time function for the 1942 Ecuador event has one simple pulse of moment release with a duration of 22 suconds, suggesting that most of the moment release occurred near the epicenter. The seismic moment determined from theP waves is 6–8×1020N·m, corresponding ot a moment magnitude of 7.8–7.9. The reported location of the maximum intensities (IX) for this event is south of the main shock epicenter. The relocated aftershcks are in an area that is approximately 200 km by 90 km (elongated parallel to the trench) with the majority of aftershocks north of the epicenter. In contrast, the 1942 Peru event has a much longer duration and higher degree of complexity than the Ecuador earthquake, suggesting a heterogeneous rupture. Seismic moment is released in three distinct pulses over approximately 74 seconds; the largest moment release occurs 32 seconds after rupture initiation. the seismic moment as determined from theP waves for the 1942 Peru event is 10–25×1020N·m, corresponding to a moment magnitude of 7.9–8.2. Aftershock locations reported by the ISS occur over a broad area surrounding the main shock. The reported locations of the maximum intensities (IX) are concentrated south of the epicenter, suggesting that at least part of the rupture was to the south.We have also examined great historic earthquakes along the Colombia-Ecuador and Peru segments of the South American subduction zone. We find that the size and rupture length of the underthrusting earthquakes vary between successive earthquake cycles. This suggests that the segmentation of the plate boundary as defined by earthquakes this century is not constant.

Great earthquakes and subduction along the Peru trench

Subduction along the Peru trench, between 9 and 15° S. involves both large interplate underthrusting earthquakes and intraplate normal-fault earthquakes. The four largest earthquakes along the Peru trench are, from north to south, the 1970 (M~ 7.9) intraplate normal-fault earthquake, and the interplate underthrusting earthquakes in 1966 (M~ = 8.0), 1940 (M = 8) and 1974 (M~ = 8.0). We have studied the rupture process of these earthquakes and can locate spatial concentrations of moment release through directivity analysis of source-time functions deconvolved from long-period P-wave seismograms. The 1966 earthquake has a source duration of 45 s with most of the moment release concentrated near the epicenter. Two intraplate normal-fault events occurred in 1963 (M~ = 6.7 and 7.0), at the down-dip edge of the 1966 dominant asperity. The 1940 earthquake is an underthrusting event with a simple source time function of 30 s duration that represents the rupture of a single asperity near the epicenter. The 1974 earthquake has a source duration of 45—50s and two pulses of moment release. This earthquake has a bilateral rupture with the first pulse of moment release located northwest of the epicenter and the second pulse of moment release located southeast of the epicenter. Both pulses of moment release occur on the northern half of the aftershock area. The 1970 earthquake is one of the largest intraplate normal-fault earthquakes to occur in a subduction zone and has a moment release comparable with many large underthrusting events. The aftershocks for the 1970 earthquake form two distinct clusters, the smaller cluster near the epicenter has focal mechanisms characterized by down-dip tension but the second aftershock cluster, located 80 km southeast of the epicenter, has focal mechanisms characterized by down-dip compression. The P-waves for the main shock can be modeled as two sources with different focal mechanisms and depths similar to the two clusters of aftershocks. The first event has a down-dip tensional focal mechanism and is followed 40 s later by a distinct second event located 80 km southeast of the epicenter with a down-dip compressional focal mechanism and a somewhat shallower depth than the first event. The observable directivity indicates that the second source is located at the second cluster of aftershocks that have down-dip compressional focal mechanisms. The occurrence of both down-dip tensional and compressional focal mechanisms may be explained by extreme ‘unbending’ stresses associated with the anomalous slab geometry. The unusually large size of the 1970 earthquake may also be related to the subduction of the Mendana fracture zone. The historic earthquake record along the Peru trench indicates that the previous event in 1746 was much larger than any of the three underthrusting earthquakes this century. The 1746 earthquake may have ruptured the entire segment in a multiple asperity earthquake. Thus, the mode of rupture along the Peru coast has changed between successive earthquake cycles.

Anomalous crust of the Bolivian Altiplano, central Andes: Constraints from broadband regional seismic waveforms

A one-year deployment of broadband seismographs in the Bolivian Altiplano recorded numerous intermediate-depth earthquakes at near-regional distances. We modeled the associated broadband waveforms of two earthquakes to estimate an average crustal structure for the Altiplano. The resulting model is characterized by an anomalously low mean P velocity of 6.0 km/s, a low Poissons ratio of 0.25, and a crustal thickness of 65 km. The combination of the low mean velocity and low Poissons ratio can be explained only by a predominantly quartz-rich, felsic bulk composition. This constraint precludes significant volumes of magmatic addition from the mantle contributing to the great thickness of the Altiplano crust, but is consistent with thickening by compressive shortening concentrated in a weak felsic layer.